Grain-oriented magnetic steel sheets used in transformers or generators are generally produced by a process in which silicon steel containing about 3% of Si is hot-rolled, subsequently cold-rolled so as to have a final sheet thickness, and then subjected to decarburization annealing (primary recrystallization annealing), followed by finishing annealing. In this process, for imparting insulation properties to a magnetic steel sheet, after the decarburization annealing and before the final finishing annealing, a slurry containing magnesium oxide is applied to a surface of the steel sheet and then dried, and wound into a coil form. Si contained in the silicon steel sheet reacts with oxygen during the decarburization annealing to form an SiO2 film on the surface of the steel sheet. SiO2 in the film then reacts with magnesium oxide in the slurry during the finishing annealing to form a forsterite (Mg2SiO4) film having excellent insulation properties on the surface of the steel sheet. The forsterite film is considered to impart not only insulation properties but also a tension to the surface thereof due to the difference in the coefficient of thermal expansion between the forsterite film and the steel sheet, thus lowering core loss of the grain-oriented magnetic steel sheet to improve the magnetic properties.
Therefore, the forsterite film plays an extremely important role in the production of grain-oriented magnetic steel sheets, and hence the properties of magnesium oxide forming the forsterite film directly affect the magnetic properties thereof. For this reason, a number of inventions have been made with respect to the magnesium oxide used as an annealing separator, especially having a controlled citric acid activity (CAA) between magnesium oxide particles and citric acid, and, for example, Japanese Prov. Patent Publication Nos. 58331/1980, 33138/1994, and 158558/1999 have been disclosed.
However, CAA merely simulates empirically the reactivity in the solid phase-solid phase reaction between SiO2 and magnesium oxide whose reaction actually proceeds on the surface of the magnetic steel sheet, based on the solid phase-liquid phase reaction between magnesium oxide and citric acid. Further, magnesium oxide is often present in the form of particle aggregate in which several powder particles bind together and agglomerate, and therefore CAA cannot appropriately evaluate the forsterite formation reaction.
On the other hand, Japanese Prov. Patent Publication No. 46259/1998 discloses an invention made in respect of the fact that the film quality varies depending on the state of the pores in magnesium oxide. In this invention, the pore volume is restricted using a constant-capacity gas adsorption method, however, in the gas adsorption method, it determines the amount of gas molecules which adsorb onto the pore surfaces present on the particle surfaces. Therefore, only pores as very small as, for example, 0.1×10−6 m or less can be measured and thus, it is considered that this method is difficult to apply to the particle aggregation structure having a size of about 1×10−5 to 1×10−6 m observed in the actual magnesium oxide particle aggregates, and hence cannot appropriately evaluate the forsterite formation reaction.
In view of this, the present inventors have found indices which can appropriately evaluate the forsterite formation reactivity of a magnesium oxide particle aggregate, and have completed an invention of a magnesium oxide particle aggregate having a particle aggregation structure specified using the indices (Japanese Patent Application No. 2000-132370). Namely, in the cumulative intrusion volume curve of the magnesium oxide particle aggregate, if these are controlled in a range where a first inflection point diameter is 0.30×10−6 m or less, an interparticle void volume is 1.40×10−3 to 2.20×10−3 m3·kg−1, and a particle void volume is 0.55×10−3 to 0.80×10−3 m3·kg−1, it is possible to form forsterite at a satisfactory rate on the surface of an magnetic steel sheet.
However, the present inventors have found that, with respect to the range considered to be unsuitable for the forsterite formation in the above earlier patent application filed by the present inventors, for example, a range in which the first inflection point diameter exceeds 0.30×10−6 m, there is a possibility that excellent forsterite formation is achieved by further strictly controlling the particle aggregation structure of the magnesium oxide particle aggregate.
An object of the present invention is to provide a magnesium oxide particle aggregate having a particle aggregation structure further controlled so that the forsterite formation rate can be appropriately controlled.
In addition, another object of the present invention is to provide an annealing separator for grain-oriented magnetic steel sheet, using the magnesium oxide particle aggregate of the present invention, and to provide a grain-oriented magnetic steel sheet obtainable by a treatment using the annealing separator for grain-oriented magnetic steel sheet of the present invention.